1. Field of the Invention
The present invention relates to a heat transport device having an evaporator and a condenser, and an electronic device. More particularly, the present invention relates to a heat transport technique that achieves size and thickness reduction by using a CPL (capillary pumped loop), a loop heat pipe, or the like in the field of fluid MEMS (micro-electro-mechanical systems).
2. Description of the Related Art
Various devices (e.g., heat pipes, heat sinks, and radiating fins) are widely used for heat radiation and cooling. Because of recent advances in the electronic device technology and micromachining technology, compact devices can be produced. For the production of such compact devices, so-called MEMS technology using a semiconductor manufacturing technology has received attention, and studies have been made to apply the MEMS technology to heat transport devices. This is because heat-source cooling systems adapted for compact and high-performance electronic devices are required, and because heat must be efficiently radiated from devices, such as CPUs (central processing units), which have markedly increased in performance, for example, processing speed.
In a capillary pumped loop, for example, the cycle in which heat is radiated from an object by vaporizing coolant in an evaporator, and the vaporized coolant is returned into liquid in a condenser is repeated, as disclosed in Jeffrey Kirshberg, Dorian Liepmann, Kirk L. Yerkes, “Micro-Cooler for Chip-Level Temperature Control”, Aerospace Power Systems Conference Proceedings (US), Society of Automotive Engineers, Inc., April 1999, P-341, pp. 233 to 238.
In a system configuration including an evaporator and a condenser, heat transport is basically conducted in the following manner:
(1) Liquid-phase working fluid fed from the condenser reaches the evaporator through a liquid channel, and is vaporized in the evaporator by heat from the outside.
(2) The vaporized working fluid flows at high speed toward the condenser through a vapor channel, and radiates heat in the condenser to return to liquid again.
(3) The above heat transport processes (1) and (2) are repeated in a closed pipe.
However, the known devices have the following problems with the heat transport efficiency and ability.
In a heat transport system including an evaporator and a condenser, when vapor in the vapor channel radiates heat and condenses into a liquid phase before reaching the condenser, the movement of the vapor is hindered by the liquid, and the heat transport efficiency may decline.
When liquid in the liquid channel is vaporized by heat from the outside before reaching the evaporator, the movement of the liquid is hindered by the vapor, and the heat transport efficiency may decline.
In an initial state at the beginning of operation, the liquid channel and a wick (serving to hold and circulate working fluid) must be filled with liquid-phase working fluid. Therefore, in order to perform stable heat transport, the cross-sectional areas of the liquid channel need to be decreased to increase the force of holding the liquid-phase working fluid by the capillary force.
In this case, if the fluid resistance increases in the liquid channel, the distance in which heat transport is possible may be decreased, and the heat quantity may be reduced. That is, in a heat transport device in which working fluid is circulated by a limited capillary force of the wick, the heat transport efficiency (the distance over which heat is transported, and the amount of heat to be transported) declines. In particular, when the device is restarted after dryout (a phenomenon in which the evaporator gets dry) is caused by excessive thermal input, working fluid is not supplied to the wick, and as a result, the heat transport device may not work.
As a solution to the problem resulting from the phase change of working fluid due to the heat exchange between the vapor channel and the liquid channel, and the outside (the flow of the working fluid is hindered, and the heat transport efficiency declines), a method is known in which heat exchange with the outside is suppressed in a large system by covering the vapor channel and the liquid channel with an insulating member. However, this method complicates the configuration of the heat transport device, and cannot be similarly applied to small heat transport devices in which channels for working fluid are provided in a substrate. In such devices that attach importance to a small and thin structure, few means have been taken to suppress the heat exchange with the outside, and therefore, the heat transport efficiency is decreased.